This invention relates to excitation current control devices for automotive electromagnetic clutches, which control the torque transmission of automotive electromagnetic clutches.
FIG. 1 is a circuit diagram of a conventional automotive electromagnetic clutch current control device. The automotive electromagnetic clutch current control device, comprising electromagnetic clutch current calculation means 2 and supplied power from a current source 3, controls the excitation current supplied to an electromagnetic clutch 4 having an excitation coil 41 and a pair of contacts 42 and 43. The electromagnetic clutch current calculation means 2, including an interface 21 and a microcomputer 22, calculates, from the running state control information S.sub.D and engine control information S.sub.E, digital instruction signal S.sub.ID for the electromagnetic clutch excitation current. A D/A converter DA then obtains an analog instruction signal S.sub.IA for the electromagnetic clutch current. A current deviation amplifier 5 obtains a deviation of the electromagnetic clutch current feedback signal S.sub.F of a current detector amplifier 6 with respect to the analog instruction signal S.sub.IA. In response to the deviation signal from the deviation amplifier 5, a PWM (pulse width modulation) modulator 7 controls the ON-OFF operation of a first output transistor 8 for controlling the current supplied to the electromagnetic clutch 4. A reflux diode 9 and a quick-break diode 11 are coupled in series with the electromagnetic clutch 4 via output terminals T1 and T2 of the automotive electromagnetic clutch current control device 1. Further, reverse excitation resistors 10 and 12 are coupled in series with the electromagnetic clutch 4 via the terminals T1 and T2 across a voltage source T3 and ground. A second output transistor 13, the emitter of which is coupled to a current detector resistor 14 and the base of which is coupled to the voltage source T3 via a base resistor 15, is always in the ON state during the time when the electromagnetic clutch 4 is in the connected state in usual operation. The second output transistor 13 is turned off in response to an electromagnetic clutch open or disconnection signal SO inputted to the base of a transistor 16 from the excitation current calculation means 2. The current detector 6, comprising an operational amplifier 61 and resistors 62 and 63, detects the voltage across the current detector resistor 14 corresponding to the excitation current through the electromagnetic clutch 4. When both the output transistors 8 and 13 are turned off, a reverse excitation current flowing in the direction opposite to that of the (normal) excitation current i.sub.c is supplied to the electromagnetic clutch 4 from the source T3 via the reverse excitation resistors 10 and 12. The reverse excitation current is necessary for compensating for and eliminating the residual torque.
As described above, the voltage across the current detector resistor 14 (which corresponds to the current flowing therethrough) is detected by the current detector amplifier 6 for the purpose of detecting the excitation current flowing through the electromagnetic clutch 4. The automotive electromagnetic clutch excitation current control device of FIG. 1 has therefore the following disadvantage.
When the output transistor 13 is being turned on, the base current I.sub.B of the transistor 13 flows through the current detector resistor 14 in addition to the excitation current i.sub.c flowing through the electromagnetic clutch 4. Further, the current I.sub.C flowing through the reverse excitation resistor 12 also flows through the current detector resistor 14. Thus, the current I.sub.E through the current detector resistor 14 is given by: EQU I.sub.E =I.sub.C .div.I.sub.B +i.sub.c
Thus, the detected electromagnetic clutch current I.sub.E includes, in addition to the actual electromagnetic clutch current i.sub.c, an error .DELTA.I.epsilon. equal to: EQU I.sub.C +I.sub.B.
For example, when I.sub.C =0.2 amperes, I.sub.B =0.1 amperes, and i.sub.c =4.0 amperes, then, I.sub.E is equal to 4.3 amperes, which thus includes an error of 7.5%. Thus, precise control of the electromagnetic clutch excitation current becomes impossible. In particular, the control of a small torque in the small current region cannot be effected with sufficient precision.
The error component due to the reverse current I.sub.C, however, may sometimes by corrected via the current detector amplifier 6. Even then, an error component due to the base current I.sub.B remains. Where I.sub.C -0.1 amperes, I.sub.B =0.2 amperes, and i.sub.c -4.0 amperes, such that I.sub.E =4.3 amperes, the base current I.sub.B of the second transistor 13 generates an error of 5%. Thus, precise control of the electromagnetic clutch excitation current becomes impossible.